1. Field of Invention
The invention relates to an external vehicle mirror and, more particularly, to an external vehicle mirror having powered extension functionality accomplished by a single motor. In another aspect, the invention relates to a slip clutch for the powered extension function.
2. Description of the Related Art
External mirrors are ubiquitous for contemporary vehicles and have long been used to aid the driver in operating the vehicle, especially in improving the rearward view of the driver. Over time, more and more functionality has been incorporated into the external mirrors. For example, it is common to extend the external mirror away from the vehicle, which is useful when towing a trailer. Mirrors incorporating both a powered fold and powered extension functionality are well-known. Examples of such mirrors are disclosed in U.S. Pat. Nos. 6,276,808 and 6,213,609, assigned to the assignee of the current application, and are incorporated by reference.
In one embodiment of the powered extension function, the mirror is extended away from the vehicle by the motor-driven rotation of an elongated drive screw which causes a threaded drive nut to travel along the drive screw. The drive nut is connected to a frame piece to which is attached the reflective element, which translates relative to the drive screw with the movement of the drive nut. Depending upon the direction of rotation of the drive screw, the mirror is either extended away from the vehicle or retracted toward the vehicle.
In operating the powered extension function, the motor is frequently operated for a preselected time interval sufficient to fully extend or retract the mirror. However, this can result in the motor continuing to operate after the mirror has reached its limit of travel. When this occurs, the motor will be prevented from turning, which can cause overworking of the motor, contributing to overheating and/or premature failure. One approach to eliminating this problem is to utilize an electronic feedback system which can determine when the motor has begun to overwork and will terminate the operation of the motor. However, these systems can be complicated, can be expensive, require additional steps in the fabrication of the mirror, and additional weight to the mirror assembly, and can themselves fail.